The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feedstocks into liquid and/or solid hydrocarbons. The feedstock (e.g. natural gas, associated gas, coal-bed methane, heavy oil residues, coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into a reactor where it is converted over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more. Examples of the Fischer-Tropsch process are described in e.g. WO 02/02489, WO 01/76736, WO 02/07882, EP 510771 and EP 450861.
Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.
As mentioned above, “coal” and heavy oil residues are examples of feedstocks for the Fischer-Tropsch process. However, there are many solid or very heavy (viscous) fossil fuels which may be used as feedstock for the process, including solid fuels such as anthracite, brown coal, bitumous coal, sub-bitumous coal, lignite, petroleum coke, peat and the like, and heavy residues, e.g. hydrocarbons extracted from tar sands, residues from refineries such as residual oil fractions boiling above 360° C., especially above 550° C., more especially above 750° C., directly derived from crude oil, or from oil conversion processes such as thermal cracking, catalytic cracking, hydrocracking etc. All such types of fuels have different levels of ‘quality’, that is the proportions of carbon and hydrogen, as well as substances regarded as ‘impurities’, generally sulphur and sulphur-based compounds, nitrogen containing compounds, heavy metals etc.
Gasification of solid carbonaceous fuels such as coal is well known, and generally involves milling or otherwise grinding the fuel to a preferred size or size range, followed by heating the fuel with oxygen in a gasifier. This creates the mixture of hydrogen and carbon monoxide referred to as syngas. However, the proportion of carbon and hydrogen in solid carbonaceous fuels is generally such that the hydrogen/carbon monoxide (H2/CO) ratio in the syngas formed is generally less than 1, whereas Fischer-Tropsch processes based on cobalt-catalysts generally desire a H2/CO ratio in the syngas to the synthesis reactor from 1.5 to 2.0, frequently 1.6-1.8. Higher ratio syngases are also desired for other parts or sections of a Fischer-Tropsch plant: some parts may desire a substantially pure hydrogen stream, that is, a very high H2/CO ratio. Further in the case that the Fischer-Tropsch process comprises two or more stages, additional hydrogen is needed between these stages. The additional hydrogen may be pure hydrogen, but, preferably, is syngas having a high H2/CO ratio.
It is an object of the present invention to increase the H2/CO ratio in the syngas derived from a range of carbonaceous fuels ready for hydrocarbon synthesis processes such as the Fischer-Tropsch reaction.